This invention relates to a method and apparatus for continuously measuring the compressive strain and determining the compressive pressure on rolls of flexible materials as they are being wound and for measuring the accumulated length of material wound on the roll.
In many industries, including papermaking and the manufacture of non-wovens, long, flexible sheets of materials are stored in continuous form by winding the material into rolls. The inherent cohesiveness of the roll and the hardness of the roll, which are developed during the winding process, provide resistance to varying loads and to shock or vibration in transit or while the roll is being unwound.
The two basic forms of winding machinery are center-winders and surface-winders. In center-winding machinery, the roll is entirely supported and driven at the core. Surface winders, which are far more common, are driven externally by one or more powered drums which contact the outside of the roll forming nips through which the material passes as it is wound. In some surface winders, at least one passive rider roll is used in conjunction with the powered drums. The rider roll and two drum configuration illustrated in the drawings is typical of surface winders used in the paper industry.
Roll cohesiveness stems from the internal stresses that are generated within the wound material. Winding results in residual tension and compressive pressure on the material wound in the roll. Ideally, there is a pressure gradient along the radius of the roll with the inner layers being more tightly wrapped than the outer layers. However, the outer layers must be wound tightly enough to provide roll cohesion and the inner layers must not be wound too tightly, or the paper in the roll may be damaged.
Excessive stretch in paper is one of the natural consequences of letting the wound-in tension rise too high. If a high enough tension is applied to a web for a long period of time and subsequently removed, the paper will never return to its original length. Cross-machine variations in the properties of the paper, such as high caliper ridges, will cause high spots in the winding roll and local increases in compressive pressure coupled with local increases in tension. When the paper is later unwound and laid flat, the excess stretched length of the paper in these ridges will cause it to form a series of weaves and puckers in trying to adapt to the less-strained and less-stretched sections of paper nearby.
If the outer layers of a roll of paper have been wound so tightly that the critical level of negative tension in the inner layers has been exceeded, buckling within the roll takes place. Visual indication of this defect is the appearance of the rosette-shaped star winding, without the roll being hit or dropped. (See, J. D. Pfeiffer, "Internal Pressures in a Wound Roll of Paper", TAPPI, Vol. 49, No. 8, p. 342-347, August 1966.).
Roll structure defects due to slippage at or near the core can cause a number of problems at the unwind station of printing presses. Rolls with very loose inner layers can slide sideways, causing a web break. A roll with a very loose core can result in a missed paster. If a roll is wound too soft near the core, it might not be able to support its own weight. See, D. Gangemi, "Rider Roll Technology of Two Drum Winders", TAPPI Finishing Conference, 1979, p. 191-196.
To avoid these roll structure defects, a number of winding control strategies have been developed. Since mathematical equations may be used to predict the compressive pressure and residual tension inside a center-wound roll after winding as a function of the ingoing tension during the winding process, one control strategy is based on measurement of the ingoing tension of a winding roll. See, H. C. Altmann, "Formulas for Computing the Stresses in Center-Wound Rolls", TAPPI, Vol. 51, No. 4, p. 176-179, April 1968; T. Rand and L. G. Eriksson, "Physical Properties of Newsprint Rolls During Winding", TAPPI, Vol. 56, No. 6, p. 153-156, June 1973; J. D. Pfeiffer, "Formulas for Calculating Roll Structure Allow Prediction Roll Defects", TAPPI Finishing Conference, 1979, p. 165-171; H. P. Yagoda, "Generalized Formulas for Stresses in Wound Rolls", TAPPI, Vol. 64. No. 2, p. 91-93, February 1981. By changing the ingoing tension, the compressive pressure and residual tension for optimum roll structure may be obtained, at least in theory. Although this strategy might be realized for center-wound rolls, it faces certain difficulties in the control of the winding process on two-drum surface winders.
It has been shown that the web tension ahead of the winder, the ingoing tension, accounts for only a portion of the total winding tension when pressure rollers or surface winding is used. Another important factor affecting total winding tension on surface winders is nip pressure of the winding drums and the rider roll. As the roll builds up, the increasing weight supported by the winding drums produces higher nip pressure, higher winding tension and a harder roll. See, J. D. Pfeiffer, "The Mechanics of a Rolling Nip on Paper Webs", TAPPI, Vol. 51, No. 8, p. 77A-85A, August 1968, which is herein incorporated by reference. There is no known technology which can measure the actual web tension after the nip in surface winders, thus roll structure control on that basis is not practical.
An alternate winding control strategy could be developed by measurement of the actual compressive pressure or compressive strain in the roll during the winding process. However, because internal compressive pressure is difficult to measure during winding, this variable cannot be used directly to control the winding process. One approach sought to overcome this problem by monitoring the density of the paper on the roll, a more easily measurable roll property which is related to the internal compressive pressure. During winding the average density of a band of paper on the roll is determined by measuring the number of layers in the band and the thickness of the band, then calculating the mean thickness of the paper and its mean density. See, L. G. Eriksson, C. Lydig, J. A. Viglund and P. Komulainen, "Measurement of Paper Roll Density During Winding," TAPPI, Vol. 66, No. 1, pp. 63-66, January 1983. The resolution of this method is limited because it depends on the number of layers used as the basis for determining the mean density, and 20 to 400 layers are required for a measurement. Moreover, this method does not take into account changes in the caliper of the paper before winding, which limits the accuracy of its measurements. These two limitations are disadvantages especially in winding control for newsprint or other papers used on printing machinery where proper roll structure is important to ensure that a roll will run properly after shipping, storage and handling.
In addition to the problem discussed above, a problem exists in measuring the accumulated length of material on a roll during winding. Sometimes, the winding process must be stopped because of the poor quality of the structure of the wound roll, e.g., a low hardness of the roll. When this happens, a number of roll layers have to be removed and the cut edges spliced before winding can be restarted. The measurement of the material length is stopped and the diameter of a roll is noted before cutting and removal of the poor layers of the roll. The length measurement is restarted after the previously noted diameter is once again obtained. In this case, an error in the length measurement is made because one cannot wind a roll the second time with the same hardness as the first time. The proper method of length measurement would be to determine the actual length of material in the wound roll after removal of the defective layers, rather than to approximate the length by measuring of the roll diameter and introducing an error by assuming identical roll structure.